The present invention relates generally to a method and arrangement for generating low-tar generator gas from a solid fuel, and more particularly to such a method and arrangement which are particularly suited for gasifying relatively low-quality solid fuels, such as bituminous solid fuels.
There is already known a multitude of methods and arrangements of the above-mentioned kind, among them such in which the solid fuel is gasified in two gasification zones by being reacted at the requisite gasification temperature with a gaseous gasification medium which at least in part contains oxygen and water vapor, and in which one part of the thermal energy needed for the gasification is obtained by combusting the solid fuel with the gasification medium, while another part of such thermal energy is supplied to the solid fuel by retrieving a part of the sensible waste heat carried out of the generator by the discharged generator gas and returning this retrieved part back into the generator by means of heated solid bodies which are admixed to the solid fuel in the gasification zone for travel therewith and discharge, together with ashes resulting from the gasification process, from the generator and subsequent separation thereof from the ashes for recycling into the solid fuel in the generator.
However, experience has shown that the heretofore known methods and arrangements of the above-mentioned kind possess the drawback that the produced generator gas is at a relatively high temperature and that, at the same time, external heat supply which is of a nature inconsistent with the gasification process is being used for the drying of the solid fuel to be gasified and/or for the pre-heating of such solid fuel, this external heat supply being made available at least in part by a partial combustion of the fuel to be gasified. In addition thereto, the ash contents of the solid fuel to be gasified constitutes a criterion for the continuous gasification of the solid fuel, so that solid fuels with low ash contents cannot be gasified in such gasifying installations.
It is already known from the Swiss Patent No. 283.414 to produce generator gas in such a manner that solid bodies are first heated in an external heating zone and then are admixed to the solid fuel to be gasified and de-gassed. The de-gassed ash-solid body mixture is then cooled by evaporation of a sprayed-in liquid. The produced generator gas is supplied to the user thereof in its cooled condition. Instead of solid fuels, as disclosed in U.S. Pat. Nos. 1,977,684 and 2,590,869, there can be used a gas, such as methane, and CO and H.sub.2 are produced by cracking processes. However, in neither one of these known developments are the solid bodies being heated by the generator gas or the CO/H.sub.2 mixture for the retrieval of the heat from such gases. In addition, the gasification medium is not fed into the apparatus of these known arrangement both above and below the generator gas discharge region, so that two separate gasification zones would be obtained in the same apparatus or generator. This brings about, in addition to an increase in the solid fuel consumption, also an increase in the required size of the installation, so that the economy of these installations is questionable especially in view of the current high prices of fuel.